1. Field of the Invention
The present invention relates to the controlling of clearance between the tops (or apices) of rotating blades and a stationary ring assembly, in a gas turbine.
2. Description of the Related Art
A gas turbine, for instance a high pressure turbine in a turbine engine, typically comprises a plurality of stationary blades alternating with a plurality of rotating blades in the passage of the hot gases exhausting the combustion chamber of the turbine engine. The rotating blades of the turbine are surrounded, on the whole periphery of the turbine, by a stationary ring assembly. Such stationary ring assembly thus defines a wall of the hot gas flow jet through the turbine blades.
In order to increase the turbine yield, minimizing the clearance between the tops of the rotating blades of the turbine and the parts of the stationary ring assembly facing these is known.
Means making it possible to vary the diameter of the stationary ring assembly have been developed for this purpose.
An example thereof can be found in document EP1555394 which provides for the air cooling of the bosses on the external annular case of said stationary ring assembly of the turbine, which has a longitudinal axis X-X. Air is injected on an external surface of the stationary ring assembly and thus causes thermal expansions or retractions of the stationary ring assembly which are able to vary the diameter thereof. <<External>> means: radially positioned outwards relative to the axis X-X. <<Radial>> means radially to the axis X-X.
Thermal expansions and retractions can be controlled according to the turbine working speed through a valve making it possible to control the flow rate and temperature of the air supplied to the ducts. The assembly consisting of the ducts and the valve thus forms a box for controlling the clearance at the tops of the blades.
Besides, the document FR 2 747 736 discloses an assembly comprising:                a gas turbine comprising an external annular case having a longitudinal axis X-X, and comprising rotating blades, and        a clearance controlling device positioned facing tops of rotating blades, which specifically comprises:                    a shroud supporting abradable ring sectors inserted radially to the shroud supporting rings, between the elastic means and the abradable ring sectors which are attached to said shroud supporting abradable ring sectors, which has a volume varying according to temperature, and            means for varying the temperature of the shroud supporting abradable ring sectors and thus for varying a clearance (j) at the tops of the rotating blades, radially to the shroud supporting rings.                        